The present invention relates to improved polyester fibers and a process for their production. More particularly, the invention relates to polyester fibers capable of being dyed under normal pressure and still having sufficient properties for practical use, and to a spinning process for preparing improved polyester fibers possessing improved stability involving spinning an extruded filament at high speed.
Polyester fiber, especially polyester fiber consisting essentially of polyethylene terephthalate, has many excellent properties such as strength and dimensional stability and many varied uses. On the other hand, polyethylene terephthalate fibers are poor in dyeability, and it is therefore necessary to dye them under the conditions of high temperature, e.g., about 130.degree. C., and high pressure. Consequently, the production of such fibers suffers from the disadvantages that a special apparatus is required for dyeing. Moreover, use of such fibers in admixture with fibers such as wool and acrylic fibers, whose physical properties deteriorate upon dyeing under high pressure and high temperature, is limited.
Various improvements in dyeability of polyester fiber under normal pressure have been proposed. A process in which carriers are employed in dyeing, for example, is known. The process has the disadvantages, however, that specific carriers are required and a subsequent treatment with dyeing liquid is difficult.
A copolymer of polyester with a compound having a metal sulfonate group or polyether has been considered a polyethylene terephthalate having improved dyeability. The dye fastness and some of the excellent properties possessed by polyethylene terephthalate inherently deteriorate in such modified polyesters, however, and it is difficult to polymerize and spin them. Consequently, the improvement resulting from such chemical modification detrimentally affects inherent properties of the fiber, as well as improving dyeability, since the improvement is achieved by introducing a third component that can act as a receptacle for dyeing the polymer.
An improvement in dyeability by other than chemical modification also has been proposed. Japanese patent publication (unexamined) No. 64133/1979, for example, discloses a flat yarn and tow prepared by spinning at a relatively high speed of about 4,000 m/min. characterized by denier per filament, intrinsic viscosity [.eta.], relative dyeing velocity of dispersed dye, modulus, modulus after boiled water treatment, amorphous modulus, shrinkage in boiled water, modulus at shrinking, and shrinkage. The Japanese patent publication discloses only polyester filament or tow having suitable properties making it suitable as a substitute for cellulose acetate fiber, however, i.e., it has a lower modulus and higher elongation than conventional polyester fibers. Furthermore, there is no description of dye fastness in the Japanese patent publication.
U.S. Pat. No. 4,134,882 discloses a polyester fiber improved in dyeability having a long period of about 300 .ANG. or more and a difference of birefringence index between that of an inner layer of fiber and that of an outer layer of fiber of less than about 10.times.10.sup.-3. The fiber is prepared by spinning at extremely high speeds of 6,000 to 8,000 yard/min. (5,400 to 7,200 m/min.). When the fiber described in the patent is dyed under normal pressure, however, it takes a long time to attain a balanced dye absorption. Thus, the fiber does not have suitable dyeability under normal pressure for commercial use.
Japanese patent publication (unexamined) No. 107511/1980 discloses a process for preparing a polyethylene terephthalate fiber having a section average birefringence (.DELTA.n) of 90.times.10.sup.-3 or more and a double structure at a section of the fiber, i.e., there is some difference in birefringence at a section of fiber between an outer layer of fiber and an inner layer thereof. Japanese patent publication (unexamined) No. 107511/1980 also points out that the fiber has mechanical properties similar to that of conventional polyethylene terephthalate fiber, viz. natural crimp and good absorption of dye. Such natural crimp, however, is insufficient for practical use and, moreover, causes lower process efficiency upon subsequent processing and a lower quality of knitted and woven fabric. Japanese patent publication (unexamined) No. 107511/1980 not only discloses and teaches a specific structure of fiber, but also a dyeability under normal pressure, and an effect related to dye fastness. Thus, the fiber of the present invention, which is capable of being dyed under normal pressure, cannot be prepared by the process described in the Japanese patent publication.
It has been disclosed that a fiber having properties adequate for practical use can be prepared by a spinning process comprising extruding a melt of polyester and spinning at high speed in Japanese patent publication (examined) No. 3104/1960, Japanese patent publication (unexamined) No. 107511/1980, and Seni Gakkai-shi 33 T208 to T214 (1977). With respect to the disclosure of polyethylene terephthalate fiber in these publications, the fiber prepared by spinning at about 5,000 m/min. or more is very similar to a conventional stretched fiber. It has been clear from studies that spinning at high speed, especially 5,000 m/min. or more, causes breaking of the filament and fiber during the spinning process and lowers spinning stability, i.e., lowers operating efficiency. This tendency increases with lower filament denier and an increase in the number of filaments.
When a polyester fiber had a filament denier of 0.5 d to 5 d and a number of filaments of 10 or more, for example, it was very difficult to spin at 6,000 m/min. or more. We have studied conditions of spinning such as viscosity of polymer, spinning temperature, spinning draw-ratio, and condition of cooling air in order to improve spinning stability at high speed spinning. We found that it was impossible to increase spinning stability by varying these conditions.
On the other hand, Japanese patent publication (examined) No. 13156/1960 discloses a process for controlling orientation of filaments by providing a heating zone substantially contacting a nozzle. Although this process permits improvement of properties such as strength and elongation, spinning stability, especially spinning stability at high speed spinning, remains a problem.
Japanese patent publication (unexamined) No. 151611/1979 discloses a process comprising subjecting a filament extruded from a nozzle to suction or a vacuum with an aspirator and then winding with a winding device. The purpose of the Japanese patent publication is to avoid raising the tension of a filament during spinning, which is caused by air resistance. A process for accumulating filaments on a conveyor net also has been known as a spun-bond process. In the process, an aspirator is employed instead of a godet roll or winder. These two processes do not teach an improvement in stability of spinning and prevention of breaking of fibers and filaments.